1. Field of the Invention
The present invention relates to guest-host liquid crystal display devices. Particularly, the present invention relates to a composition of dichroic dye which is added as a guest to a nematic liquid crystal functioning as a host.
2. Description of the Related Art
A guest-host liquid crystal display devices includes a nematic liquid crystal and dichroic dye dissolved in the nematic liquid crystal, in which alignment of the dichroic dye, as well as the nematic liquid crystal, is controlled by an applied electric field. Such dichroic dye shows anisotropic light absorption, and light transmittance of a liquid crystal display device is controlled by such a phenomenon. Guest-host liquid crystal display devices are classified into a transmitting type and a reflecting type. Dichroic dye showing a black color is used for monochrome displaying of black and white in these two types. A combination of the guest-host liquid crystal display device and a micro color filter provides a full color guest-host liquid crystal display device. Accordingly, the dichroic dye must be a black dye suitable for monochrome displaying. It is, however, difficult to achieve monochrome displaying with only one dye, hence a mixed-type black dye composed of different color dyes is generally used. The requirements for dichroic dye include (1) high dichroic ratio, (2) large absorption coefficient, and (3) high compatibility with or solubility in liquid crystals. A higher dichroic ratio causes a higher contrast because of high anisotropy in light absorption. A higher absorption coefficient enables a sufficiently high contrast with a small amount of dichroic dye. Further, a higher solubility of dichroic dye in the liquid crystal enables a satisfactorily high contrast even at a lower temperature, since it barely precipitates at a lower temperature.
Black dyes are prepared by mixing, for example, yellow dyes, orange dyes, red dyes, bluish purple dyes and blue dyes. Among these dyes, there are many types of yellow dyes, orange dyes and red dyes, and some of them satisfy the above-mentioned three requirements. In contrast, only a few bluish purple dyes and blue dyes have been known, and these bluish purple dyes or blue dyes barely satisfy the three requirements. Bluish purple dyes and blue dyes have absorption bands from about 550 nm to 620 nm for which the human eye is sensitive and thus greatly affect the contrast. Conventionally used bluish purple dyes and blue dyes include tris-azo dyes and anthraquinone dyes. The anthraquinone dyes do not satisfy the absorption coefficient and the compatibility with the liquid crystal among the three requirements. Although the tris-azo dichroic dye has a relatively high balance between these requirements, it is still unsatisfactory in view of practical use. That is, a large amount of tris-azo dichroic dye relative to the liquid crystal is required because of a low absorption coefficient. As a result, the dye easily precipitates as a crystal at a low temperature, and operational characteristics of the liquid crystal deteriorate since the dye functions as an impurity. In recent years, active matrix liquid crystal display devices, in which thin film transistors are used for pixel switching, have been widely used, and such devices require stable operational characteristics.